The performance of a proton-exchange membrane (PEM) fuel cell and a direct methanol fuel cell (DMFC) is largely determined by the membrane-electrode assembly (MEA). The MEA is generally composed of an anode that oxidizes the fuel, a cathode that reduces oxygen, and an ionically-conducting membrane for proton conductance. The membrane also prevents electrical short-circuiting between the anode and the cathode, and it physically separates the fuel from the oxidant.
A PEM fuel cell normally uses hydrogen as the fuel, while a DMFC uses methanol. As used herein, the term “fuel cell” represents both types of fuel cells.
The fuel oxidation reaction and the oxygen reduction reaction are kinetically slow, and therefore, catalysts such as platinum and its alloys are used to catalyze the fuel cell reactions. The catalysts are made into porous layers to increase the contact area between the reactants and the catalyst particles. The catalyst materials can be applied to the membranes, or upon gas diffusion media. Carbon paper and carbon cloth-type materials are the prevalent gas diffusion media owing to their good electrical conductivity, high corrosion resistance, and controllable porosity.
One approach toward producing low cost fuel cells is reducing the amount of noble metal materials used in the catalyst layers. Metal black with lower surface area was initially tried. The catalyst loading in an electrode needs to be over 4.0 mg/cm2 in order to achieve good performance. Later attempts utilized smaller metal nano-particles with a higher surface area. The nano-particles were deposited upon a support, as illustrated in U.S. Pat. No. 4,166,143, issued to Petrow et al on Aug. 28, 1979 for CONTROL OF THE INTERACTION OF NOVEL PLATINUM-ON-CARBON ELECTROCATALYSTS WITH FLUORINATED HYDROCARBON RESINS IN THE PREPARATION OF FUEL CELL ELECTRODES; U.S. Pat. No. 4,876,115, issued to Ian D. Raistrick on Oct. 24, 1989 for ELECTRODE ASSEMBLY FOR USE IN A SOLID POLYMER ELECTROLYTE FUEL CELL; and Re. 33,149, issued to Petrow et al on Jan. 16, 1990 for FINELY PARTICULATED COLLOIDAL PLATINUM COMPOUND AND SOL FOR PRODUCING THE SAME AND METHOD OF PREPARATION OF FUEL CELL ELECTRODES AND THE LIKE EMPLOYING THE SAME.
The catalyst support has several functions. It provides sites for anchoring the metal particles during and after their formation. The particles formed in such an environment have small and uniform size. In addition, due to the chemical/physical interaction between the support and these metal particles, the latter tend not to coalesce or aggregate, thus becoming less likely to lose surface area as fast as an unsupported catalyst. In addition, the support provides electrical connection for metal particles supported on different support particles. Carbon black is the most practical support used in fuel cells because of its large surface area, good electrical and thermal conductivities, and high corrosion resistance.
Electrical as well as proton conductivities are needed, since both electrons and protons are involved in fuel cell reactions. Thus, the reaction zone is traditionally limited within the interface between the catalyst layer and the ionically-conducting membrane. Since this interfacial region is extremely thin, the total surface area of the catalyst particles in this region is low; thus, such a catalyst layer cannot provide a high current density. The catalyst that is not in contact with the membrane is simply wasted.
More recently, an ionic conductor such as Nafion®, a perfluorinated ionomer, made by E. I. DuPont, has been incorporated into the catalyst layers. After Nafion incorporation, the entire catalyst layer conducts both electrons and protons, dramatically improving the performance of the catalyst layer. The catalyst layer is thus able to generate and sustain higher current densities.
Nafion can be impregnated into a catalyst layer by brushing and spraying, or by floating the electrode upon, or dipping it in, a Nafion solution, as illustrated by Ticianelli et al, J. Electrochem. Soc. pp. 2209–2214 (1988), September; and by Poltarzewski et al, J. Electrochem. Soc. pp. 761–765 (1992), March. The advantage of this procedure is that a water-repelling agent like polytetrafluoroethylene (PTFE) can be incorporated into the catalyst layer before the application of Nafion, so the final catalyst layer has a controllable hydrophobicity. This reduces the likelihood of flooding in the fuel cell. The disadvantage of this method, however, is that it is very difficult to control the amount of the applied Nafion. It is impossible to have a homogeneous distribution of Nafion in the entire catalyst layer. The regions with more Nafion may become flooded, while the regions with insufficient Nafion may not be able to provide enough proton conductivity. Nafion may localize on the surface in one place, but penetrate the underlying gas diffusion medium in another place.
Another method to incorporate Nafion into the catalyst layer is to mix catalysts (especially supported catalysts) with Nafion directly. The resulting mixture is then used to make the catalyst layer, as illustrated in U.S. Pat. No. 5,211,984, issued to Mahlon S. Wilson on May 18, 1993 for MEMBRANE CATALYST LAYER FOR FUEL CELLS; U.S. Pat. No. 5,723,173, issued to Fukuoka et al on Mar. 3, 1998, for METHOD FOR MANUFACTURING SOLID POLYMER ELECTROLYTE FUEL CELL; U.S. Pat. No. 5,728,485, issued to Watanabe et al on Mar. 17, 1998 for ELECTRODE FOR POLYMER ELECTROLYTE ELECTROCHEMICAL CELL AND PROCESS OF PREPARING SAME; and U.S. Pat. No. 6,309,772 B1, issued to Zuber et al on Oct. 30, 2001 for MEMBRANE-ELECTRODE UNIT FOR POLYMER ELECTROLYTE FUEL CELLS AND PROCESSES FOR THEIR PREPARATION.
The catalyst and Nafion can form a pretty good mixture using these patented methods. Nafion and the catalyst materials achieve a more even distribution through the entire catalyst layer. Solvents such as glycerol may be used during the mixing in order to achieve a good viscosity, and additionally, to hold the catalyst particles in suspension. This minimizes their agglomeration, as shown in aforementioned U.S. Pat. No. 5,211,984. Sometimes, a Nafion solution is converted into a colloid by adding a proper organic solvent, before mixing it with the catalyst, as illustrated in aforementioned U.S. Pat. No. 5,723,173, in which the colloidal Nafion is claimed to form a good network and achieve a uniformity upon distribution of the catalyst particles.
Directly mixing a Nafion solution or colloid with the catalyst makes it difficult to incorporate PTFE into the catalyst layer. This is so because the subsequent sintering of the PTFE at a temperature higher than 330° C. destroys Nafion. Without PTFE, the catalyst layer is more likely to be flooded, and therefore must be thin.
Although the incorporation of Nafion has dramatically increased the efficacy of catalyst in the catalyst layer, a large portion of the catalyst particles still do not participate in the reactions. Nafion molecules are fairly large, with a molecular weight on the order of 250,000 to 1,000,000 g/mole. They only can penetrate into the gaps among the agglomerates of the carbon support, and each one of the agglomerates possibly contains hundreds of primary carbon particles. It is obvious that most of the primary carbon particles will not be in contact with Nafion particles, so they cannot participate in the reactions.
Catalyst particles formed within the pores of the carbon support have little chance to participate in the reaction, even though the carbon particles themselves are in direct contact with the Nafion particles, because Nafion particles cannot get into the pores at all. Of course, for the carbon particle agglomerates that are not mixed with Nafion particles, none of the supported catalyst particles will be able to contribute.
It will be difficult for the reactants to enter, or for the products to leave, those catalyst particles that are buried under Nafion. Thus, their activities will be reduced. In addition, the incorporation of Nafion into a catalyst layer will reduce the gas permeability and electronic conductivity of the catalyst layer, which will in turn reduce the fuel cell performance. This is especially so when the cell is operated at high current densities.
It has been envisioned that the catalyst utilization will be further increased if a proton conductor is directly linked onto the surface of the carbon support. Some initial work has been done toward achieving this goal. Prepared polymer composites like polypyrrole/polystyrenesulfonic acid and poly(3,4-ethylenedioxythiophene)/polystyrenesulfonic acid have been used as catalyst supports. These composite particles conduct both electrons (by the conducting polymer such as polypyrrole, and poly(3,4-ethylenedioxythiophene)) and protons (by the ionomer, polystyrenesulfonic acid). Thus, good oxygen reduction has been achieved, as illustrated by Qi et al, Chem. Commun., pp. 15–16, 1998; Chem. Commun., pp. 2299–2230, 1998; and J. Electroanal. Chem., pp. 9–14, 1998; and by Lefebvre et al, J. Electrochem. Soc., 146, pp. 2054–2058, June 1999. A major problem with using a conducting polymer as the support is the loss of electronic conductivity under fuel cell operating conditions.
Jia et al teach that increasing the wettability and proton conductivity of the surface of carbon particles by oxidizing the carbon with nitric acid and other oxidants, produces enhanced electrode performance (Electrochimica Acta, pp. 2863–2869, 2001). However, since carboxylic acid is a weak acid and a poor proton conductor, the increased performance is very limited. Easton et al recently attached sulfonated silane onto the surface of carbon support and achieved the optimal fuel cell performance with only 10% of Nafion (dry mass) mixed into the catalyst layer. It was demonstrated that 30% of Nafion was needed to achieve the best performance for the unsulfonated counterpart, as illustrated in Easton et al, Electrochem. Solid-State Letters, pp. A59–A61, May 2001. However, the best performance of the sulfonated silane-treated catalyst was not as good as that of the untreated counterpart. This inferior performance could be due to the reduction of electronic conductivity of the carbon support and the blockage of some catalyst particles by the attached silane. In addition, the silanization process is not easy to control.
One object of the present invention is to provide an effective and simple procedure to link sulfonic acid onto the surface of carbon supporting a catalyst using sulfonation agents.
Another object of this invention is to increase the proton conductivity at the surface of carbon particles supporting the catalyst.
Still another object of this invention is to increase catalyst utilization within a catalyst layer so a higher performance can be achieved with a reduced amount of catalyst loading.
Still another object of this invention is to reduce the amount of Nafion needed in a catalyst layer.